An heir will become ruler of a side immediately upon the ruler croaking or abdicating. If the new ruler's side owns at least one city, then the side continues as before. If it has no cities of its own, then the side falls and the new ruler becomes a barbarian.
It is not uncommon for an heir to also serve as Chief Warlord, but it is not necessary for one unit to serve both functions, and both titles can be redistributed by the ruler.
The heir status doesn't carry over if the unit changes their side.Erf-b3-p174
A side can have multiple heirs at once. The ruler picks one of them to be the primary heir, who will become the default heir to receive the side in the event the current ruler falls or abdicates.Erf-b3-p153
Popping a Heir
Heirs are always popped as the production of the capital. The process takes a lot of turns (60 turns for Faq after TBFGKErf-b2-pP16). The ruler doesn't have to be in the capital when it's started.Erf-b2-pE20 The ruler getting croaked doesn't stop this production automatically either.Hvs.tCF 219
Just like popping any other unit, the process can be sped up significantly by a Turnamancer.
The popped heir is always considered and refered to as a son or a daughter of the ruler.
Promotion to Heir Designate rarely occurs and is very expensive.Erf-b1-p079 It costed roughly 165,000 Schmuckers to name Prince Tramennis Heir Designate (5K for each of the 28 Archons + 25K bonus for capturing one, assuming Gobwin Knob had not lost or gained any archons in between the Battle for Gobwin Knob and the Battle of Expository Bridge.)
- Gobwin Knob
- Goodminton - Side destroyed
- Haffaton - Side destroyed
- Don King - Former popped heir to King County; croaked
- Prince Ponzie - Former popped heir to Don King, tried to usurp the throne; croaked
- Unknown Royal female - Former popped heir to Don King; croaked
- Viscount Caesar Borgata - former Noble heir designate to Don King; now Regent
- Unknown Royal Heir, being popped.
It may be the case that all Heirs must be Commanders; every Heir seen thus far was either popped as a Warlord-Heir, or was named Heir Designate while serving as a Warlord or Caster. (Stanley was promoted to Warlord, and then named Heir Designate.) Naming an Heir Designate may require promoting the unit to Warlord first.
If a side has a heir and they're popping another heir, most probably the new heir will be the primary heir by default.